In this and other contexts a ratchet may be engaged by a latch to provide a multiplicity of possible settings of a position of one member that carries the ratchet relative to another member which carries the latch. An ordinary ratchet and latch provides one-directional travel of one component past another. The latch may be sprung so that its profile engages discrete positions of the profile of the ratchet. Movement in the reverse direction is prevented by the locking engagement of the two profiles. The backlash of the system is determined by the pitch of the discrete locking positions of the engagement profile between the ratchet and the latch. It is, therefore, possible for the moving part of the system to come to rest within an infinitesimally small distance from which it can engage with a discrete locking position of the fixed component. From this position, there is nothing to prevent a reversal of travel up to the previous discrete locking position, which is approximately one pitch away from the desired resting position. It is, therefore, advantageous to minimize the backlash of the system such that reverse movement is efficiently reduced.